1. Field of the Invention
The present invention relates to an arrangement in a wall in a fluidized bed reactor. By means of such an arrangement, a very reliable and abrasion-resistant wall is achieved.
The present invention also relates to a method of coating in a fluidized bed reactor a wall. By means of such a method, a long-lasting abrasion-resistant coating for the wall is achieved.
2. Description of the Related Art
The abrasion of various walls is a harmful phenomenon in many industrial applications. Various transport systems, such as pneumatic fuel conveyors and reactors in which various mixtures are treated, are often subjected to abrasion, even hard such abrasion during their operation. Especially gas suspensions containing solids can cause excessive abrasion. The pneumatic transport, for instance, of solid fuel, such as coal and peat, subjects the parts of the transport channels to hard abrasion. Many points of discontinuity, bends or elbows cause local abrasion points in the flow system.
Also, in fluidized bed reactors in which the fluidizing velocity is so high that the reactor functions as a so called circulating fluidized bed reactor, the abrading effect of the circulating material on the walls of the reactor, especially at certain points, has been found to be a serious problem.
Fluidized bed reactors are used in a variety of different combustion, heat transfer, chemical and metallurgical processes. Depending on the process, various bed materials are fluidized and/or circulated in the system. In combustion processes, the fluidized bed consists mainly of particulate fuels, such as coal, coke, lignite, wood, wood waste, coal waste or peat as well as other particulate matter, such as sand, ash, sulphur absorbent, catalysts or metal oxides.
A fluidized bed reactor generating heat comprises an upright reactor chamber having substantially vertical outer walls. The walls are made as waterwalls, i.e., tube walls in which vertical tubes are connected to each other by flat plate material or "fins". The walls in the lower part of the reactor are usually refractory lined so as to withstand heat and erosion. The violent turbulence and abrading effect of the particles and the relatively high density of solid material result in highly erosive conditions, particularly in the bottom part of the reactor.
In certain regions of the reactor, there are both downward and upward flows of bed material. The absolute mass flow varies in radial as well as in axial directions of the reactor chamber. The downward mass flow is at its biggest near the outer wall. As the density of particles increases in the downward direction in the rector chamber, the layer of particles flowing down along the outer walls also thickens. The downward flowing layer can have a thickness of up to 10-50 mm, or even greater. Any change in the flow direction of the downward flowing layer causes erosion.
The upper edge of the refractory lining in the waterwall construction forms a shoulder in the reactor chamber, which causes a vortex in the downward flowing layer of bed material. The direction of the layer flowing vertically downwards along the fins connecting the tubes is partly changed, whereby the layer begins to flow along the edge of the refractory lining. The vortex and the horizontal flow of particles along the edge cause heavy erosion of the waterwall tubes, especially in those parts that are close to the lining. The erosion is in particular problematic in boilers using solid fuel and in which the conditions are highly conducive to erosion.
The tubes in the waterwall have to be inspected from time to time and, if needed, be recoated with protective material or replaced by new tubes. Extensive shutdown time is required to cut off the damaged tubes and to install new ones or to renew the protecting surface. Both alternatives are difficult, expensive and time consuming processes.
While the problem with erosion of tubes in fluidized bed reactors is well known and various solutions have been suggested to minimize erosion, such solutions have not been entirely successful. A lining shielding the tubes higher up in the reactor would decrease the erosion but it would at the same time also decrease the heat transfer to the tubes.
Welding a layer of protective material on the tubes or coating them in some other way in particularly vulnerable regions has been tried. The welds do not, however, last for a very long time in highly erosive surroundings. It has also been suggested to decrease the velocity of the flow along the tube walls by welding studs or other obstacles on the walls, which would decrease the flow rate of the particles along the surface of the tubes. A high velocity enhances, however, the heat transfer at the tube walls and the velocity should, therefore, not be decreased. In published Swedish Patent No. 454,725, it has been suggested to weld curved segments at especially hard wearing locations.
In the arrangement according to published Swedish Patent No. 452,360 the entire reactor walls are inclined inwardly in the upward direction to decrease erosion of the walls. This is a very peculiar construction and not easily accomplished.
In published Finnish Patent Appln. No. 913314 it has been suggested to bend the tube wall outwardly in the downward direction in the region between the uncovered part and the refractory-lined part thereof, at an angle to the vertical plane. The tube wall is then either bent back to the vertical at a distance downwardly from the first bend or it may be bent inwardly at an angle so as to form an inclined inner wall in the combustion chamber. This construction does not form a shoulder, which the lining of the vertical waterwall usually does, wherefore this construction allows a falling layer to flow along the tubes without any vortexes forming in the flow of particles. Bending the tubes first outwardly and then back is a troublesome task and slows down the production process of the boiler wall. In addition, in existing boilers, such an arrangement is both difficult to accomplish and expensive. In rectangular reactor chambers, this arrangement causes additional problems, especially at the corners.
It has been suggested to coat the tube wall of a boiler above the lining of the lower part by metal spraying the tube wall up to a certain level. This has, for instance been suggested in the patent specifications Japanese document No. (JP) 4-198461, Japanese document No. (JP). In these arrangements, the coating will always be to some extent porous by the technique commercially available at present, which increases the susceptibility to abrasion of the coating, especially in the area where it begins, in which either a distinct shoulder is produced or the coating is gradually faded out.
Efforts have been made to decrease the susceptibility to wear by raising the coated area higher and higher in the reactor, where the particle density is smaller than in its lower part and the abrading effect is consequently smaller. This arrangement is, however, very expensive and obviously also increases the heat transfer resistance in a large area.
Another arrangement which has been suggested for increasing the strength of the sprayed coating is to fade out the coating gradually over a relatively long distance so as to prevent the forming of a distinct discontinuity. Even this arrangement has proved to be inadequate. A very thin layer at the beginning of the fading-out section will easily come off from the surface resulting in even greater wearing and loosening.
It has further been suggested in U.S. Pat. No. 3,988,239 a replaceable liner for a cyclone whose inner surface defines a longitudinal passageway through the body. The liner body comprises a hollow bladder with an internal circumferential recess and a generally tubular insert disposed in the recess. The insert defines the greater wear portion of the passageway and is adapted to withstand such greater wear. Preferably the linear material is of flexible nature to facilitate replacement of the wear resistant insert.